Plant Physiology - Musienko, M. M. 2001

General principles of regulation of plant growth and morphogenesis
Gibberellins

The discovery of this group of phytohormones is associated with The Study of the rice disease known as "bakanae" or foolish seedling disease, which is characterized by intense elongation of the stem and leaves. In 1926, a team of Japanese scientists led by E. Kurosawa investigated this widespread rice seedling disease caused by the fungus Gibberella fujikuroi. They isolated a fungal extract that proved to be the active substance and named it gibberellin. In 1954, B. Cross proved that the gibberellin isolated from the fungus was actually a mixture of active compounds. He managed to isolate a third active compound from this mixture, which proved to be the most active and was named gibberellic acid GA₃. In 1956, gibberellin was also isolated from the seeds of the common bean (Phaseolus vulgaris). Later, gibberellins were isolated from other species of higher plants. The synthesis of GA₁ proceeds as follows:

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Gibberellins are a large group of compounds belonging to the class of tetracyclic diterpenoids, a complex family of plant secondary metabolites related to Lipids. They are weak carboxylic acids featuring a "gibberellane" Skeleton (Fig. 179).

Fig. 179. Gibberellane skeleton, gibberellic acid, and some other gibberellins

Today, more than 90 different gibberellins are known, designated by the abbreviation GA. To distinguish them, they are assigned nomenclature numbers such as GA₁, GA₂, and so on, while gibberellic acid itself is known as GA₃. All of them share the basic tetracyclic gibberellane skeleton, but each possesses distinct modifications and chemical groups.

Not all gibberellins possess physiological activity. Most bioassays for gibberellins are based on:

· stimulation of SHOOT growth in genetically dwarf varieties of pea, corn, and rice;

· stimulation of growth in lettuce or cucumbers;

· increase in α-amylase activity in the endosperm of cereal seeds (barley, rice).

They are found in Fungi, Algae, and higher plants, with immature seeds exhibiting the highest concentrations. Different stages of plant ontogeny may be characterized by varying levels and sets of gibberellins. Although gibberellins are synthesized in various PARTS OF THE plant Organism, the leaves serve as their primary site of production. Light stimulates The formation of gibberellins. It is likely that gibberellins exist in both free and bound forms. Storage and transport forms occur as GA Glycosides. They move from the leaves both upward and downward, being transported via the xylem or phloem streams. Generally, they travel through young Tissues, such as shoot and ROOT apices. The direction of gibberellin movement in germinating cereal seeds has been precisely established: from the scutellum to the Cells of the aleurone layer. Unlike Auxins, gibberellins do not exhibit polar transport.

Physiological Action of Gibberellins

The action of gibberellins is most clearly manifested in their ability to stimulate stem elongation when applied to dwarf mutants of maize (Fig. 180).

Fig. 180. Gibberellin bioassay using dwarf maize. A gibberellin solution was applied as droplets to the primary leaf of dwarf plants, which was folded into a cap-like shape. Left: a normal (non-dwarf) plant of the same species for comparison. Numbers indicate The amount of GA₃ in μg

The causes of plant dwarfism vary. Genetic dwarfism is caused by changes at the Gene level and may result from impaired gibberellin synthesis. As a rule, dwarfism manifests as a reduction in internode length alongside an increase in their number. Dwarf plants treated with gibberellin match the height of normal plants, although the dwarf trait is preserved in subsequent generations. Gibberellins significantly enhance stem elongation in many normal plants as well (by approximately 30–50%). For this reason, gibberellin is considered the stem Growth Hormone. Stem elongation is driven primarily by Cell expansion rather than Cell Division, with apical and intercalary Meristems serving as the sites of action.

The Effect of gibberellins on the transition to the flowering phase in long-day plants is closely linked to stem growth and The Emergence of plants from the rosette stage (bolting). The most striking manifestation of their action is the stimulation of bolting and flowering in long-day plants under short-day conditions; in short-day plants, gibberellin may even induce the opposite effect regarding flowering (Fig. 181).

Gibberellins stimulate flowering in numerous plant species, representing the only known group of substances with this activity. The dose of exogenous gibberellin ranges from 3 to 100 μg per plant, depending on the species.

Exogenously applied gibberellin eliminates the requirement for vernalization in biennial plants. As is well known, biennial plants grow during their first year but do not flower. Only after exposure to low temperatures during the winter period and an optimal photoperiod do they transition to flowering in their second vegetation year. This induction or stimulation of flowering in vegetative plants under METABOLISM/18.html">The Influence of low temperatures is called vernalization. However, gibberellin is unable to replace the effect of low temperatures and induce flowering in biennials that do not form rosettes but instead develop stems during their very first year of growth.

Fig. 181. Effect of gibberellin on cabbage flowering (left — control, right — gibberellin)

They are also capable of breaking the dormancy of seeds and tubers in certain plants. Exogenously administered gibberellin replaces stratification in seeds that require it. Treatment of seeds from many plant species with gibberellin accelerates their germination.

Gibberellic acid induces parthenocarpy (The Development of fruits without Fertilization and, consequently, without seed formation), such as in stone fruits. Gibberellins do not stimulate root growth, and at elevated concentrations, they are even inhibitory.

One can hypothesize two pathways by which gibberellins control specific physiological processes: first, simply through the synthesis of gibberellins by plants followed by the initiation of a gibberellin-dependent process; second, recall that there are over 90 GAs that differ in their relative activity depending on the process they influence. Because they all share a similar Structure, they are readily interconverted. This allows the plant to easily regulate various metabolic processes by converting an inactive GA into an active one and vice versa. Gibberellin activates the synthesis of Nucleic Acids and Proteins. The way GA controls starch Hydrolysis in barley grains lacking an embryo has been studied in considerable detail. This control boils down to regulating the production and release of Enzymes, as applying GA to such grains triggers the appearance of amylase and Other Enzymes. Thus, under the influence of GA, Messenger RNA encoding The production of specific enzymes is synthesized.

Like other phytohormones, GA interacts primarily with a proteinaceous photoreceptor. It has been proven that GA activates enzymes controlling phospholipid synthesis, the Formation of the granular Endoplasmic reticulum (ER), and enzyme secretion.

Previous data also indicate that gibberellin induces cell elongation through Cell wall acidification, in a manner similar to auxin. Moreover, it turned out that GA-sensitive cells do not respond to auxin. This may be explained by the presence of different Hormone Receptors in them. Gibberellins stimulate pollen germination, similarly to auxin, and accelerate fruit development. Treatment of grapes with gibberellins results in the formation of larger berries and a looser cluster:

Like auxin, gibberellins undergo enzymatic degradation, the products of which are currently unknown.